As the number of applications for electro-optical systems increases, the role of microlens arrays (MLAs) to allow adequate performance to cost is crucial and an effective method to mass produce MLAs need to be developed. Several methods that have been used to fabricate MLAs include; thermal reflow and etching transfer; hot embossing; and laser ablation. However, there remain drawbacks to the use of these methods including the fabrication cost. For example, the thermal reflow and etching transfer method uses a complicated lithographic procedure that further suffers from the generation of environmentally unfriendly wastes. The hot-embossing method remains impractical because of shrinkage and thermal damage problems. Laser ablation methods remain impractical for most mass production due to high fabrication costs. Another common method is to create a master mold by methods such as machining and electroplating, followed by filling the mold with a monomer or oligomer, polymerizing the monomer or oligomer, and attaching the resulting MLA to the desired device. This method requires multiple steps for MLA fabrication and attachment and, thus, the fabrication costs are higher than they would be for a single step method.
One area where the use of a MLA can significantly lower the cost of a technology is the deposition of such an array on a light emitting diode (LED) including an organic light emitting diode (OLED). In general, OLEDs consist of a multi-layer sandwich of a transparent electrode, such as indium-tin-oxide (ITO), as anode contact, one or more organic layers including a light emitting molecule or polymer layer, and a metal layer as cathode, deposited on a planar substrate, usually of glass or a high refractive index plastic such as polycarbonate. In “bottom emission” devices, the transparent electrode, usually the anode, is deposited on the front surface of a transparent substrate followed by a multilayer sandwich, with the reflecting electrode layer, usually the cathode, furthest away from the substrate. Light generated internally in the light emitting layer is transmitted out of the device through the transparent electrode and the substrate. Conversely, in a “top emission” device, the reflective electrode is in contact with the substrate while the transparent electrode is furthest away from the substrate. The light generated internally in the light emitting layer is then coupled externally through the transparent electrode layer without passing through the substrate. Usually the transparent electrode layer is the cathode which can be made substantially transparent by using a transparent conducting material or a thin metal layer having a thickness less than around 50-100 nm. In bottom emission devices, typically only about 20% of generated light is emitted through the substrate-air interface with the remainder being trapped and absorbed within the substrate, e.g. 35%, and the light emitting multi-layer sandwich and the transparent electrode, e.g. 45%.
This low light emitting efficiency arises primarily because light is wave-guided, scattered and reflected internally at the layer interfaces due to the different refractive indices exhibited by the different materials foiining the layers of the device. Efforts have been directed to improve the coupling efficiency, by affording the back side of the substrate with a non-planar profile, or by attaching microlenses, in order to extract more light from the substrate. For example, Sturm et al. WO 01/33598 discloses patterning the back side of the substrate in the shape of a sphere centered on the multi-layer light source by attaching a molded sphere to the back surface of the substrate or by shaping the back surface of the substrate into a spherical form. Thus, some of the generated light that would otherwise be reflected internally at the substrate-air interface escapes the substrate, thereby increasing the amount of light emitted from the device.
According to WO 01/33598, the total emitted light can be increased by a factor of up to 3, and the normal emitted light can be increased by a factor of nearly 10, through the use of spherical lenses of various radii of curvature on glass or polycarbonate substrates of various thicknesses. The disclosed lenses have a radius of curvature (R) to substrate thickness (T) ratio (R/T) in the range from 1.4 to 4.9. Similarly, Kawakami et al. JP-A-9171892 discloses that light emission from the substrate can be increased by including a spherical lenses shape on the emitting face of the substrate in which the radius of curvature (R) to substrate thickness (T) ratio (R/T) is about 3.6. Smith et al. WO 05/086252 discloses improvements in light emission from an OLED device by forming or attaching spherical microlenses to the substrate, at the substrate-air interface, such that the radius of curvature (R) to substrate thickness (T) ratio (R/T) is in the range from 0.2 to 0.8. Each microlens is disposed on the front surface of the light coupling layer where the microlens extends across the full width of each pixel. Smith et al. disclose that the lenses may be applied to or formed in the light coupling layer, by adhering the lenses to the light coupling layer, by directly embossing the light coupling layer or substrate to form the appropriately shaped lenses, or by adhering an embossed laminate having the lenses formed therein to the light coupling layer.
A cost effective method for placing microlenses on a LED or OLED would encourage a wider use of this technology for devices that include flat-panel displays and solid-state lighting because of the improved light emitting efficiency possible due to the microlenses. One method of forming a microlens on a substrate that has been investigated is via ink-jet printing.
Inkjet printing is based on the phenomena that a fluid under pressure issues from an orifice, typically 50 to 80 μm in diameter, and breaks up into uniform drops when a capillary wave is induced onto the jet, usually by an electromechanical device that causes pressure oscillations to propagate through the fluid. The drops break off from the jet in the presence of an electrostatic field, referred to as the charging field, which imparts an electrostatic charge to the drops. The charged drops are directed to their desired location, either a catcher or one of several locations on a substrate, by another electrostatic field, referred to as a deflection field. This type of system is generally referred to as “continuous” because drops are continuously produced and their trajectories are varied by the amount of charge applied. Continuous mode ink-jet printing systems produce droplets that are approximately twice the orifice diameter of the droplet generator.
The drops can also be produced by electro-induced pressure waves in a fluid under ambient pressure where a volumetric change in the fluid is induced by the application of a voltage pulse to a piezoelectric transducer, which is directly or indirectly coupled to the fluid. This volumetric change causes pressure/velocity transients to occur in the fluid and these are directed to produce a drop that issues from an orifice. Since the voltage is applied only when a drop is desired, these types of systems are referred to as “drop-on-demand” (DOD). A thin film resistor can be substituted for the piezoelectric drive transducer where the fluid in contact with the resistor is vaporized to form a vapor bubble over the resistor upon passing a high current through the resistor. This vapor bubble serves the same functional purpose as the piezoelectric transducer. Demand mode ink-jet printing systems produce droplets that are approximately equal to the orifice diameter of the droplet generator.
Ink-jet printing allows a precise microdispensing of a fluid in a repeatable manner. The droplets generated by current DOD device may be varied in diameter from about 15 μm to about 120 μm by changing the dispensing device orifice diameter and/or the drive waveform at rates up to about 25,000 drops per second. Piezoelectric demand mode does not create thermal stress on the fluid and does not depend on the thermal properties of the fluid to impart acoustic energy to the working fluid which enables the dispensing of fluids ranging from polymer formulations to liquid solders. Appropriate viscosities of many fluid formulations can be optimized by a controlled heating of the fluid to a desired temperature. Ink-jet printing has become a key enabling technology in the development of bio-MEMS devices, displays, sensors, electrical components, and micro-optical systems. More recently ink-jet printing has been explored for use for opto-electronic packaging. It has been explored for printing microlenses for optical interconnects including optical fiber collimators, solders for electrical interconnects, and adhesives for bonding and sealing.
Inkjet printing has been used to form microlenses on substrates. Microlenses have been formed by the deposition of a drop of a polymer in solution where the microlens is formed upon the removal of the solvent. Additionally, microlenses have been formed by the deposition of drops of monomers or polymers with functionality that can be polymerized on a substrate by thermal or photochemical means, for example as disclosed in Hayes, U.S. Pat. No. 6,805,902. Such systems require that the resulting microlens is well attached to the substrate. For LED and OLED applications, it is desirable that a microlens have a large contact angle with a substrate to optimize the proportion of light transmitted from the device. The typical substrate droplet interface displays a contact angle that is considerably less than 90 degrees.
To promote a large contact angle the surface of the substrate has been patterned with a coating as disclosed in Huang et al, Poster Presentation P-MST53 at MNE'06, http://dimesnet.dimes.tudelft.nl/mneabstracts/P-MST/P-MST53.pdf. The substrate is unmodified on the area for deposition of the microlens but coated with a low surface energy material such that the deposited material resists spreading over the coated portion of the substrate, enhancing the contact angle between the coated substrate and the microlens material but does not compromise the more a robust attachment of the microlens to the uncoated substrate.
The modification of the surface by deposition of a patterned coating significantly complicates the process, as the drop must be specifically directed to the uncoated portions of the surface. The requirement of such specific drop positioning requires a precision that significantly increases the cost of the microlens formation process. Hence, a simple cost effective method to form a microlens with a high contact angle between the microlens and the substrate that can be easily integrated with existing OLED processes remains a goal for the development of OLED devices.